1. Field of the Invention
The present invention relates to an image scanning/recording apparatus and method for forming a continuous band-shaped image of a uniform width by simultaneously applying a plurality of beams to a medium to be exposed.
2. Description of the Related Arts
A high speed recording is one of the important characteristics required for an image recording apparatus such as a laser printer. In order to accomplish a high speed recording, it is conceivable to carry out method of exposing a plurality of beams simultaneously. As one of methods for obtaining a plurality of beams, an apparatus includes a beam splitter which divides a beam into a plurality of beams by utilizing reflections of a plurality of glass plate surfaces having a plurality of surfaces parallel with a laser light source. The intensities of beams divided by the beam splitter are not equal to each other. Accordingly, it is preferable to modulate the optical intensities of beams in Switching Spots. The rise time of an optical shutter should be less than 1.mu.sec for forming an image at a high speed.
An acoustooptic element is utilized as an optical-modulating element having the function of adjusting the optical intensities of a plurality of beams and allowing the optical shutter to rise at a high speed. A multichannel-applicable acoustooptic element can be used to independently modulate beams generated by the beam splitter. The acoustooptic element utilizes the phenomenon of Bragg diffraction which is caused by a diffraction grating generated by an acoustic wave in a material thereof. The diffraction angle of a beam is determined by a diffraction grating interval and wavelength of the beam. The diffraction grating interval is determined by the speed of an acoustic wave in an acoustooptic material and the frequency thereof. Accordingly, supposing that the wavelength of light incident on the acoustooptic element is constant, the diffraction angle of the light depends on the frequency of the acoustic wave applied to the acoustooptic material. The acoustooptic element used in a conventional image scanning/recording apparatus such as a laser printer functions only as an optical shutter but may be regarded as having a scanning function.
When divided beams of single natural light emitted by a laser light source or light linearly polarized in one direction and emitted by a laser are superimposed on each other, a clear spot image is not formed on a medium to be exposed due to the interference of beams. That is, if a continuous band-shaped scanning line is recorded by using such beams, interference fringes appear in the spot image. As a result, light and dark irregularities appear periodically in a band-shaped recorded pattern. Accordingly, divided beams of natural light or divided beams of light linearly polarized in one direction cannot be superimposed on each other. That is, in order to form an image by natural light having a high intensity, it is necessary to adopt a exposing/scanning method of generating a plurality of beams sequentially in exposure of a medium to the beams.
As described in Japanese Laid-Open Patent Publication No. 1-237615, in a conventional method for scanning/recording an image, beams linearly polarized at a right angle with each other are used to eliminate the effect of interference between adjacent beams. FIG. 3 shows an apparatus, for carrying out the method, disclosed in the above Patent Publication. A laser beam emitted by a gas laser 21 is divided into two kinds of beams, i.e. a change-direction beam and a straight-ahead beam, by a beam splitter 22. A beam reflected approximately 90.degree. by the beam splitter 22 with respect to a beam which has linearly passed through the beam splitter 22 is incident on a polarization beam splitter 27 via a beam splitter 31, a mirror 26, and an acoustooptic element 25. The beam splitter 31 divides a beam into a plurality of parallel beams. The acoustooptic element 25 modulates a plurality of parallel beams independently, thus emitting a recording beam row to be used to record an image. The beam which has passed through the beam splitter 22 linearly is incident on the polarization beam splitter 27 via a beam shifter 23, a beam splitter 24, an acoustooptic element 25, and a mirror 26. The beam splitter 24 and the acoustooptic element 25 have the same functions as those elements 24 and 25, respectively. The two kinds of beams which have been incident on the polarization beam splitter 27 are condensed by a lens 28, thus forming four spots 30a, 30b, 30a, and 30b on a medium 29 to be exposed. A beam which forms the spot 30a and a beam which forms the spot 30b are different from each other in the direction of polarization plane thereof. According to this method, since the beams are linearly polarized, only less than 50% of the output power of natural light emitted from the laser light source is used to record an image on the medium. In addition, one multichannel-applicable acoustooptic element 25 is required to generate each of the two kinds of beams which are incident on the polarization beam splitter 27. Therefore, two multichannel-applicable acoustooptic elements 25 are required.